The present invention is generally related to the fabrication of integrated circuits (ICs) and, more specifically, to the fabrication of a highly stable conductive electrode barrier using a composite film including iridium, oxygen, and a transition metal, with an adjacent barrier including a transition metal.
Platinum (Pt) and other noble metals are used in IC ferroelectric capacitors. The use of noble metals is motivated by their inherent chemical resistance. This property is especially desirable under high temperature oxygen annealing conditions, such as those seen in the fabrication of ferroelectric capacitors. In addition, chemical interaction between noble metals and ferroelectric materials such as perovskite metal oxides, is negligible.
The above-mentioned noble metals are used as conductive electrode pairs separated by a ferroelectric material. One, or both of the electrodes are often connected to transistor electrodes, or to electrically conductive traces in the IC. As is well known, these ferroelectric devices can be polarized in accordance with the voltage applied to the electrode, with the relationship between charge and voltage expressed in a hysteresis loop. When used in memory devices, the polarized ferroelectric device can be used to represent a "1" or a "0". These memory devices are often called ferro-RAM, or FeRAM. Ferroelectric devices are nonvolatile. That is, the device remains polarized even after power is removed from the IC in which the ferroelectric is imbedded.
There are problems in the use of metal, even noble metal electrodes. Pt, perhaps the widely used noble metal, permits the diffusion of oxygen, especially during high temperature annealing processes. The diffusion of oxygen through Pt results in the oxidation of the neighboring barrier and substrate material. Typically, the neighboring substrate material is silicon or silicon dioxide. Oxidation can result in poor adhesion between the Pt and neighboring layer. Oxidation can also interfere with the conductivity between neighboring substrate layers. Silicon substrates are especially susceptible to problems occurring as a result of oxygen diffusion. The end result may be a ferroelectric device with degraded memory properties. Alternately, the temperature of the IC annealing process must be limited to prevent the degradation of the ferroelectric device.
Various strategies have been attempted to improve the interdiffusion, adhesion, and conductivity problems associated with the use of noble metals as a conductive film in IC fabrication. Titanium (Ti), titanium oxide (TiO2), and titanium nitride (TiN) layers have been interposed between a noble metal and silicon (Si) substrates to suppress the interdiffusion of oxygen. However, Ti layers are generally only effective below annealing temperatures of 600 degrees C. After a 600 degree C. annealing, Pt diffuses through the Ti layer to react with silicon, forming a silicide product. Further, the Pt cannot stop the oxygen diffusion. After a high temperature annealing, a thin layer of silicon oxide may be formed on the silicon surface, which insulates contact between silicon and the electrode.
Other problems associated with the annealing of a Pt metal film are peeling and hillock formation. Both these problems are related to the differences in thermal expansion and stress of Pt with neighboring IC layers during high temperature annealing. A layer of Ti overlying the Pt film is known to reduce stress of the Pt film, suppressing hillock formation.
Ir has also been used in attempts to solve the oxygen interdiffusion problem. Ir is chemically stable, having a high melting temperature. Compared to Pt, Ir is more resistant to oxygen diffusion. Further, even when oxidized, iridium oxide remains conductive. When layered next to Ti, the Ir/Ti barrier is very impervious to oxygen interdiffusion. However, Ir can react with Ti. Like Pt, Ir is very reactive with silicon or silicon dioxide. Therefore, a bilayered Ir/Ti or Ir/TiN barrier is not an ideal barrier metal.
Co-pending application Ser. No. 09/263,595, entitled "Iridium Conductive Electrode/Barrier Structure and Method for Same", invented by Zhang et al., and filed on Mar. 5, 1999, discloses a multilayered Ir/Ta film that is resistant to interdiffusion.
Co-pending application Ser. No. 09/263,970, entitled "Iridium Composite Barrier Structure and Method for Same", invented by Zhang et al., and filed on Mar. 5, 1999, discloses a Ir composite film that is resistant to interdiffusion, and highly stable during high temperature annealing.
Co-pending application Ser. No. 09/316,646, entitled "Composite Iridium Barrier Structure with Oxidized Refractory Metal Companion Barrier and Method for Same", invented by Zhang et al., and filed on May 21, 1999, discloses a Ir composite film with an oxidized transitional metal barrier layer that is resistant to the interdiffusion, and highly stable during high temperature annealing.
It would be advantageous if alternate methods were developed for the use of Ir as a conductor, conductive barrier, or electrode in IC fabrication. It would be advantageous if the Ir could be used without interaction to an underlying Si substrate.
It would be advantageous if an Ir film could be altered with other conductive metals to improve interdiffusion properties and structural stability. Further, it would be advantageous if this improved type of Ir film could be layered with an interposing film to prevent the interaction of Ir with a silicon substrate.
It would be advantageous if the above-mentioned Ir-metal film could resist the interdiffusion of oxygen at high annealing temperatures. It would also be advantageous if the Ir-metal film was not susceptible to peeling problems and hillock formation.
It would be advantageous if the Ir-metal film could be produced which remains electrically conductive after annealing at high temperatures and oxygen ambient conditions.
Accordingly, a highly temperature stable conductive barrier layer for use in an integrated circuit is provided. The barrier comprises an underlying silicon substrate, a first barrier film including a refractory metal barrier overlying the substrate, and an iridium-refractory metaloxygen (Ir--M--O) composite film overlying the first barrier film. Typically, the first barrier film is selected from the group of materials consisting of Ta, Ti, Nb, Zr, Hf, TiN, TaN, NbN, ZrN, AlN, and HfN. The first barrier layer has a thickness in the range of approximately 10 to 100 nanometers (nm). The first barrier prevents the interdiffusion of Ir into the Si or SiO2 substrate. It also prevents other reactions between the first composite film and the substrate.
The Ir--M--O composite film remains conductive after high temperature annealing processes in an oxygen environment. Further, the Ir--M composite film resists hillock formation, and resists peeling. Specifically, the Ir composite film includes the following materials: Ir--Ta--O, Ir--Hf--O, Ir--Ti--O, Ir--Nb--O, Ir--Al--O, and Ir--Zr--O. Typically, the Ir--M--O composite film has a thickness in the range of approximately 10 to 500 nm. The above-mentioned transition metals, as well as oxygen, are added to the first composite film to stuff the Ir polycrystalline grain boundaries to improve its stability, and to resist barrier penetration and hillock formation.
In some aspects of the invention, a second barrier film including a noble metal overlies the first barrier layer, to limit the diffusion of oxygen into the first composite film. The second barrier helps prevent the oxidation of the first barrier during annealing processes. The second barrier film is selected from the group of materials consisting of iridium oxide (IrO2), ruthenium oxide (RuO2), Ir, platinum (Pt), and ruthenium (Ru).
In some aspects of the invention a third barrier film, including a noble metal, overlies the Ir--M--O composite film. The third barrier material is selected from the group of materials including IrO.sub.2, RuO.sub.2, Ir, Ru, and Pt. The third barrier improves the interface between the first composite film and the subsequently deposited film, such as a ferroelectric film, improving crystal orientation, and improving the leakage current and fatigue characteristics of the subsequently formed ferroelectric device.
In some aspects of the invention, the barrier is used as an electrode in a ferroelectric device. Then, a ferroelectric film overlies the Ir--M--O film. A conductive metal film made of a noble metal, of the above-mentioned Ir--M composite film, or of other multilayered conductive top electrode overlies the ferroelectric film. The ferroelectric film is capable of storing charges between the top and Ir--M--O electrodes.
Also provided is a method for forming a highly temperature stable conductive barrier overlying an integrated circuit substrate. The method comprising the steps of:
a) through PVD (physical vapor deposition), CVD (chemical vapor deposition), or MOCVD (metal organic chemical vapor deposition) processes, depositing a first barrier layer, as described above, overlying the substrate; PA1 b) through PVD, CVD, and MOCVD processes, depositing a first composite film including iridium, a transition metal, and oxygen, as described above, overlying the first barrier layer to a thickness in the range of approximately 10 to 500 nm; and PA1 b.sub.1) annealing the first composite film in an atmosphere selected from the group of gases consisting of oxygen, N.sub.2, Ar, and a vacuum, and in which the annealing temperature is in the range of approximately 400 to 1000 degrees C., whereby the conductivity of the first composite film is improved and the thickness of the first composite film is stabilized. PA1 a.sub.1) through CVD, PVD, or MOCVD processes, depositing a second barrier layer including a noble metal, as described above, overlying the first barrier layer. The second barrier layer resists the diffusion of oxygen into the first barrier film. PA1 c) (optional) using PVD, CVD, and MOCVD deposition processes, depositing a third barrier layer, of the above-described noble metal materials, over the Ir--M--O composite film to a thickness in the range of approximately 10 to 200 nm, improving the first composite film interface to subsequently deposited materials; PA1 d) depositing a ferroelectric material overlying the first composite film; and PA1 e) depositing a conductive top electrode overlying the ferroelectric material, whereby a ferroelectric capacitor is formed.
In some aspects of the invention, a further step follows Step a), of:
In some aspects of the invention, wherein a ferroelectric capacitor is formed, a further step follows Step b), of:
Sputtering is one PVD process used to deposit the composite and barrier films. For example, the first barrier material can be deposited from a 4 inch diameter targets by sputtering in Step a) at a power in the range of approximately 50 to 800 watts, in an Ar atmosphere at a pressure of 2-100 millitorr (mT). Step b) can include cosputtering Ir and a metal targets with a power level in the range of approximately 50 to 800 watts. The metal targets are selected from the group of metals consisting of Ta, Ti, Nb, Zr, Al, and Hf. The atmosphere is Ar--O.sub.2 with a flow ratio in the range of approximately 1:5 to 5:1, and the atmosphere pressure is in the range of approximately 2 to 100 mT. When metal nitride is to be deposited, Step a) includes establishing an Ar--N.sub.2 atmosphere.
Alternately, Step b) includes depositing the first composite film through PVD deposition, sputtering with a single, composite source, in an oxygen environment. The single composite source material is selected from the group of materials consisting of Ir, Ta, Ti, Nb, Zr, Al, Hf, and oxides of the above-mentioned materials.